An agarose gel electrophoretic method has been developed to separate the different species of RNA and ribosomes obtained from mouse liver. The polyribosomes can be readily separated into their components, from monomer to hexamer. Treatment of polyribosomes with RNAse produced monosomes with a faster mobility in electrophoresis than that of naturally-occurring monosomes. In vivo binding of C14-dimethylnitrosamine (DMN) to rRNA in C3H mice was studied. The radioactivity was found predominantly in the region of 28S RNA. The ratio of radioactivity between 28S rRNA and 18S rRNA was much higher with DMN than with methionine.